Titanium alloy and method for heat treatment of large-sized semifinished materials of said alloy

ABSTRACT

The inventive titanium alloy comprises, expressed in mass %: aluminium 4.0-6.0; vanadium 4.5-5.0; molybdenum 4.5-5.0; chromium 2.0-3.6; ferrum 0.2-0.5; the rest being titanium. An equivalent molybdenum content is determined as corresponding to Mo equiv. ≧13.8. The total aluminum and zirconium content does not exceed 7.2. The inventive method for heat treatment consists in heating to t β&lt;&gt;α+β −(30-70)° C., conditioning during 2-5 hrs. at that temperature, air or water cooling and age-hardening at a temperature ranging from 540° C. to 600° C. during 8-16 hrs. Said alloy has a high volumetric deformability and is used for manufacturing massive large-sized forged and pressed pieces having a high strength level, satisfactory characteristics of plasticity and fracture toughness.

FIELD OF THE INVENTION

[0001] The inventions relates to non-ferrous metallurgy, and moreparticularly, to production of modern titanium alloys preferably usedfor manufacturing of large-sized forgings, stampings, fasteners andother parts for aeronautical engineering.

PRIOR STATE OF ART

[0002] Titanium-based alloy of the following composition, % by mass, isknown: aluminum 4.0-6.3 vanadium 4.5-5.9 molybdenum 4.5-5.9 chromium2.0-3.6 iron 0.2-0.8 zirconium 0.01-0.08 carbon 0.01-0.25 oxygen0.03-0.25 titanium the balance

[0003] (RF Patent # 2122040, C22C 14/00, 1998) as the prototype.

[0004] The said alloy possesses a good combination of high strength andplasticity of large-sized parts up to 150-200 mm thick, water or airhardened. The alloy is easily hot deformed and is welded by argon-arcand electron-bean welding.

[0005] The disadvantage of the alloy is an insufficient level ofstrength of massive large-sized parts more than 150-200 mm thick, airhardened.

[0006] The method of heat treatment of large-sized semifinished itemsmade of two-phase titanium alloys comprising pre-heating up to thetemperature 7-50° C. higher than the polymorphic transformationtemperature, holding for 0.15-3 hours, cooling to the two-phase regiontemperature, 20-80° C. lower than the polymorphic transformationtemperature, holding for 0.15-3 hours, hardening and aging is known(USSR Inventor's Certificate # 912771. C22F, Jan. 18, 1982) as theprototype.

[0007] The disadvantage of the method is an insufficient level ofstrength of massive large-sized parts more than 150-200 mm thick.

DISCLOSURE OF THE INVENTION

[0008] An object of the claimed titanium-based alloy and method of heattreatment of large-sized semifinished items of the said alloy is toattain higher level of strength of massive large-sized parts 15-200 mmin excess thick.

[0009] The integral technical result attained in the process ofrealization of the claimed group of inventions is the regulation ofoptimal combination of α- and β-stabilizing alloying elements in theproduced semifinished item.

[0010] The said technical result is attained by the fact thattitanium-based alloy containing aluminum, vanadium, molybdenum,chromium, iron, zirconium, oxygen and titanium additionally containsnitrogen, with the following distribution of components, % by mass:aluminum 4.0-6.0 vanadium 4.5-6.0 molybdenum 4.5-6.0 chromium 2.0-3.6iron 0.2-0.5 zirconium 0.7-2.0 oxygen no more than 0.2 nitrogen no morethan 0.05 titanium the balance

[0011] while the molybdenum equivalent Mo_(JKβ)≧13.8.

[0012] According to the invention the molybdenum equivalent isdetermined by the following relation: $\begin{matrix}{{Mo}_{jK\beta} = {\frac{\quad {\% {Mo}}}{1} + \frac{\% \quad V}{1.5} + \frac{\% \quad {Cr}}{0.6} + \frac{\% {Fe}}{0.4}}} & (1)\end{matrix}$

[0013] Besides, total content of aluminum and zirconium does not exceed7.2 (2)

[0014] The said technical result is attained also by the fact that inthe method of heat treatment of large-sized semifinished items of theclaimed titanium-based alloy comprising heating, holding at thetemperature lower than the polymorphic transformation temperature,cooling and aging, in accordance with the invention heating is performeddirectly to t_(β⇄α+β)−(30-70)° C., holding at the said temperature isperformed for 2-5 hours, and aging is performed at 540-600° C. for 8-16hours. Cooling is performed in air or water.

[0015] Mostly β-phase is responsible for high strength of the alloy dueto the sufficiently wide range of β-stabilizers (V, Mo, Cr, Fe), theirconsiderable amount and efficiency of their ability to affect thepossibility of maintaining the meta-stable phase condition duringretarded cooling (for instance, in air) of massive cross-sectionstampings. Though β-phase is the leading one in the process of the alloystrengthening, it is possible to enhance the tendency to strengthincreasing only at the expense of strength increase of α-phase, normalfraction of which for this alloy is 60-70%. To do this, alloying ofα-phase with α-stabilizing zirconium was intensified; the latter forms awide range of solid solutions with α-titanium, is relatively close to itin terms of melting temperature and density, it increases corrosionresistance and in quantity up to 1.5-2.0% softly increases the alloystrength, and practically does not decrease its plasticity and crackingresistance.

[0016] Due to the regulation of β-stabilizers in the form of molybdenumequivalent according to relation (1) with establishing of its minimalvalue, increasing of the zirconium content and regulation of theα-stabilizers content in accordance with relation (2), in combinationwith optimization of processing to solid solution parameters, includingheating and holding at the temperature lower than the polymorphictransformation temperature, massive articles of the claimed alloy afterair (or water) hardening from the processing to solid solutiontemperature, have after the aging step higher level of strength withsatisfactory plasticity and destruction viscosity characteristics.

[0017] This application meets the requirement of unity of invention asthe method of heat treatment is intended for manufacture of semifinisheditems of the claimed alloy.

EMBODIMENTS OF THE INVENTION

[0018] To study the alloy characteristics test 430 mm diameter ingots ofthe following average composition were manufactured: TABLE 1 Chemicalalloy Mo_(OKB) Alloy Al Mo V Cr Zr Fe Ti β

α + β t° C. (Al + Zr) 1 5.2 5.0 5.1 3.0 0.01 0.4 the 840 14.4 5.21balance 2 5.1 4.9 5.3 3.1 1.2 0.35 the 845 14.5 6.3 balance

[0019] The ingots were forged in series in β, α+β, β, α+β-regions withfinish deformation in α+β-region in the range of 45-50% per 250 mmdiameter cylindrical billet.

[0020] Further the forgings were subjected to the following heattreatment:

[0021] a) Processing to solid solution: heating at 790° C., holding for3 hours, cooling in air.

[0022] b) Aging: heating at 560° C., holding for 8 hours, cooling inair.

[0023] Mechanical properties of the forgings (averaged data in per unitdirection) are given in Table 2. TABLE 2 σ_(0.2)(VTS), σ_(B)(UTS), δ(A)ψ(Ra), K_(1C) Alloy MPa (KSi) MPa (Ksi) % % MPa {square root}{squareroot over (M)} (KSi {square root}{square root over (in)}) 1 1213 (176)1304 (189) 12 36 53.2 (48.4) 2 1255 (182) 1350 (195.6) 10.5 33 51.5(46.85)

[0024] The test results show that the claimed alloy and the method ofheat treatment of semifinished items of it permit to ensure more secureand stable increase of strength characteristics of massive parts whilemaintaining satisfactory plasticity characteristics.

Commercial Practicability

[0025] The claimed group of inventions is intended for any articles(rods, forgings, plates, etc.) but particularly for massive forgings andstampings (with in excess 150-200 mm side dimension or cross-sectiondiameter, wherein it is required to ensure high level of strength.

1. Titanium-based alloy containing aluminum, vanadium, molybdenum,chromium, iron, zirconium, oxygen and titanium which distinction is thatit additionally contains nitrogen with the following proportion ofcomponents, % by mass: aluminum 4.0-6.0 vanadium 4.5-6.0 molybdenum4.5-6.0 chromium 2.0-3.6 iron 0.2-0.5 zirconium 0.7-2.0 oxygen no morethan 0.2 nitrogen no more than 0.05 titanium the balance

while the molybdenum equivalent Mo_(JKβ)≧13.8.
 2. Alloy as claimed inclaim 1 which distinction is that the molybdenum equivalent isdetermined by the following relation:${Mo}_{jK\beta} = {\frac{\% \quad {Mo}}{1} + \frac{\% \quad V}{1.5} + \frac{\% \quad {Cr}}{0.6} + \frac{\% \quad {Fe}}{0.4}}$


3. Alloy as claimed in claims 1 and 2 which distinction is that totalcontent of aluminum and zirconium does not exceed 7.2.
 4. Method of heattreatment of large-sized semifinished items of titanium-based alloyscomprising heating, holding at the temperature lower than thepolymorphic transformation temperature, cooling and aging whichdistinction is that heating is performed directly to t_(β⇄α+β)−(30-70)°C., holding at the said temperature is performed for 2-5 hours, andaging is performed at 540-600° C. for 8-16 hours.
 5. Method as claimedin claim 4 which distinction is that cooling is performed in air or inwater.